It has long been recognized that obstructions upstream of a gate, in the molding of tubular parts, such as parisons, about a core or pin, can result in flow lines in the part. In the case of a parison, the flow lines are carried to the blown part. Such flow lines are caused by the parting of the plastic material, such as a polymer, as it flows around an obstruction, such as a core-support pin, positioned upstream of the orifice or gate. At the least, flow lines result in poor appearance of the blown piece. However, such lines can severely impair the strength of a finished part. Strength impairment is particularly critical in the forming of parisons from thermoplastic polymers which have high melt viscosities and which, when parted, do not readily flow or mix together, or rejoin, in a laminar state. While the finished part may present only a slight marring when physically examined, with many such polymer materials it has been found that the finished part is also severely weakened along these lines and can lead to premature failure. Such premature failure is particularly critical in blown automotive bellows and boots which are subject to mechanical as well as environmental stresses.
The problem is particularly acute in the manufacture of boots and bellows and the like from parisons which are formed about cores, where the latter are supported by conventional radially extending core support pins at the inlet end of the mold. There accordingly exists a need for apparatus and method for the remixing of such polymer material on the downstream side of such core support pins to eliminate such weld lines and lines of imperfection or weakness in the completed part.